Over the past several years, many different methods have been proposed in the integrated circuit processing art for forming sub-micron images. Most of these methods rely on state-of-the-art photolithographic tooling. Other methods rely on more exotic exposure systems (e.g., X-ray, E-beam, etc.). Whichever of the above exposure systems are used, its source intensity, beam focus, and other parameters combine to establish a minimum feature size that can be reliably printed.
Recently, efforts have been made to extend the useful life of optical tooling that produces images that are too large for present-day applications. One method of supplementing these minimum images is by the use of sidewall structures. In this technology, a conformal layer is coated on a "mandrel," which is a block of material typically having substantially vertical sidewalls. The conformal layer is then etched in an anisotropic mode, so that portions thereof laying on horizontal surfaces (e.g., the upper surface of the mandrel) are removed. Portions of the conformal layer disposed on vertical surfaces (e.g., the sidewalls of the mandrel) remain to form the sidewall spacers. In some applications, the sidewall-coated mandrels are used as a mask to define an image in underlaying layers. See an article by Varshney, entitled "Self-Aligned VMOS Structure Using Reactive Ion Etching," IBM Technical Disclosure Bulletin, Vol. 22, No. 8, January 1980, pp. 3705-3706, wherein an oxide mandrel is coated with a conformal oxide layer that is etched to define oxide spacers, the mandrel-spacer composite defining an image in underlaying silicon layers. In other applications, the mandrel is removed, without removing the spacers, and the spacers define an image. See e.g, U.S. Pat. No. 4,502,914 entitled "Method of Making Structures with Dimensions in the Submicrometer Range," issued Mar. 5, 1985 to Trump et al and assigned to the assignee of the present invention. A conformal layer of silicon oxide or silicon nitride is coated on a polymeric mandrel. After spacers are defined on the sidewalls of the mandrel, the mandrel is removed and the spacers serve as masks to etch deep trenches in the underlaying silicon substrate.
As shown by the above references, the respective compositions of the conformal layer and the mandrel are determined by the particular masking application of the resulting structure. In applications in which the mandrel-spacer combination serves as a mask, both materials must be resistant to the etchant that patterns the underlaying layers. In applications in which only the spacer is used as a mask, the mandrel must be made of a material that can be etched without appreciably attacking the spacers.
The above references utilize silicon oxide and silicon nitride as the conformal layers that define the sidewall spacers. Both of these materials are typically deposited at temperatures of 175.degree. C. and above. Organic mandrel structures (e.g., photoresist) may begin to reflow at these high deposition temperatures. Moreover, when these layers are deposited on oxide or nitride passivation layers, such layers may be etched during definition of the spacers. In the above referenced U.S. Pat. No. 4,707,218, this shortcoming was addressed by incorporating an etch-stop layer on the passivation layer.
Accordingly, it would be advantageous to use the same mandrel-spacer combination for each of the above applications, while avoiding the high deposition temperatures and overetch problems attendant with conventional conformal layers.